In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters or images. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the recording medium, non-contact with printed characters or images, etc. In recent years, in order to impart good weathering resistance and good water resistance to printed materials, an ink containing a pigment as a colorant has been extensively used.
On the other hand, it has been required to print characters or images on recording media for commercial printing purposes using a low-liquid absorbing coated paper such as an offset-coated paper, or a non-liquid absorbing resin film such as a polyvinyl chloride resin film, a polypropylene resin film and a polyester resin film.
It is known that when characters or images are printed on the low-liquid absorbing or non-liquid absorbing recording media by the ink-jet printing methods, there tends to occur problems such as prolonged drying time owing to slow or no absorption of liquid components, and deterioration in rub fastness early after printing. Also, it is known that unlike a plain paper in which a pigment is likely to be penetrated, the low-liquid absorbing or non-liquid absorbing recording media tend to suffer from deposition of pigment particles remaining thereon which are directly susceptible to an external force, so that the characters or images printed on these recording media tend to be deteriorated in rub fastness even after being dried.
In order to solve these conventional problems, ink-jet printing methods using recording media having an ink-absorbing layer have been proposed.
For example, JP 2008-260279A discloses an ink-jet printing method using a recording medium having a pigment-containing coating layer in which an amount of pure water transferred to a surface of the recording medium on which the coating layer is provided, and a pH value of the same surface of the recording medium are controlled to respective specific ranges, and characters or images are printed on such a surface of the recording medium using an ink containing a granular coloring material, an emulsion resin and a surfactant and having a pH value of not less than 8. In addition, JP 2008-260279A proposes a recording apparatus equipped with a drying device.
However, the improvement in recording media and recording apparatus tends to pose problems concerning costs, consumed power, etc., and it has been therefore required to improve the composition of inks.
Further, for the purpose of stably compounding a pigment into a water-based ink, colored fine particles prepared by encapsulating the pigment with a polymer have been developed.
For example, JP 2003-306611A discloses a microencapsulated pigment capable of producing an ink for ink-jet printing which can satisfy all of requirements (1) to (6) including (1) excellent dispersion stability; (2) excellent ejection stability from a print head; (3) capability of procuring printed materials having images with excellent fastness; (4) capability of procuring printed materials having images with high optical density; (5) capability of procuring printed materials having images with excellent scratch resistance; and (6) capability of procuring printed materials having images with high color developability which hardly suffer from ink bleeding even when using a plain paper as a recording medium. The microencapsulated pigment includes pigment particles that are coated with a polymer having a repeating constitutional unit derived from a polymerizable surfactant containing a hydrophilic group, a hydrophobic group and a polymerizable group.
In addition, JP 2003-261605A discloses a sulfuric acid salt containing an alkylene group having 5 to 18 carbon atoms or an alkoxymethylethylene group, and an alkylene group having 2 to 4 carbon atoms, as a polymerizable surfactant capable of providing a polymer emulsion that is kept stable during polymerization thereof and provides a polymer coating film having good properties such as water resistance. The polymer emulsion described in Patent Literature 3 can be used as a paint or an adhesive in the form of the polymer coating film.